It is well known to treat substrates such as wood with polyurethanes or alkyd resins. Polymers are generally applied either neat or from solvent solution as one-part or two-part systems. Frequently, isocyanate terminated polymers are used. The toxicological problems associated with free monomeric isocyanates are well known. It is desirable to make a coating where the end user is not exposed to free monomeric isocyanates.
In addition, the use of solvents creates problems of pollution, toxicity, and flammability and increases the cost of formulating and processing polyurethane materials. Solvents, however, are often required in the preparation and handling of polyurethane resins to sustain a controllable and processible viscosity.
Many of the known references emphasize polyurethane materials which are unstable to water and are generally kept from contact with water until after application to the surface being treated, e.g., leather. Thus, U.S. Pat. No. 3,179,713 (Brown) describes the surface treatment of leather with polysiloxanes containing isocyanate radicals as terminal groups. These are employed in amounts of 10-75% by weight with triorganosilyl endblocked diorganopolysiloxanes. The resulting product has all the characteristics of a siloxane-treated leather, except that the reactive isocyanate groups are stated to provide better bonding. Such compositions must be applied from solvent and must be protected from exposure to moisture prior to application to the leather. The compositions are applied at 15% to 25% by weight of the leather.
A class of moisture-curable silyl group-containing polymers is described by Brode et al., U.S. Pat. No. 3,632,557. The polymers are described as "vulcanizable" and are formed as films and plaques which cure by exposure to atmospheric moisture. Because of this sensitivity, the patentee taught that care had to be exerted at all times to avoid premature exposure to moisture. A polyurethane sealant containing alkoxysilyl terminating groups is described by Seiter, U.S. Pat. No. 3,627,722.
Latex polymers which are vinyl addition polymers formed by free radical polymerization and comprise vinyl hydrolyzable silane, an ester of the group of acrylic, maleic and fumaric esters and/or vinyl acetate are described by Kowalsli et al. in U.S. Pat. No. 3,814,716. These are dispersed in water using anionic or nonionic surfactants and are said to be useful to give durable coatings on various substrates which are generally rigid. It is known that the introduction of surfactants to such systems enhances the hydrophilicity of latices stabilized in this manner, leading to a reduced moisture resistance and surface adhesion of such coatings.
Gaa et al., U.S. Pat. No. 4,582,873 describes a process for making an aqueous dispersible, polyurethane polymer which has internal pendant, siliconate anions. The aqueous dispersion is prepared from a reaction product of polyisocyanates, organic compounds with at least 2 active hydrogens, a hydrophilic additive, and organosilane which is at least monofunctional, preferably difunctional, in reaction with isocyanate groups on at least one organic moiety of the organosilane and also has at least one hydrolyzed or hydrolyzable groups associated with the silicone atom. The hydrophilic additive, which is present at a level of up to about 10 weight percent of the prepolymer or polymer depending on the hydrophilicity of the polyisocyanates employed, assists in promoting the emulsification and stability of the disclosed aqueous dispersions. The aqueous dispersion of the polyurethane resin is used in coating a variety of substrates such as inorganic oxide substrates.
Gaa, European Patent Appl. 0305833 B1, discloses a silane terminated hydrophilic material based on vinyl alcohol and copolymers thereof as well as sugars, which may be a solution or dispersion.
U.S. Pat. No. 5,041,494 (Franke et al.) discloses a cationically stabilized silane terminated polyurethane made with hydrophilic polyether compounds and, optionally, externally added alcoholic, aminic and/or hydrazinic chain lengthening agents. These polyurethanes are made via a volatile organic solvent process, wherein the volatile organic solvent is introduced early in the polyurethane prepolymer formation and which must be stripped from the final product via an additional step.
One means for reducing the moisture sensitivity that arises due to the addition of these dispersant stabilizers in polyurethane dispersions, such as nonionic, cationic, and anionic surfactants and other hydrophilic additives, is through the use of external crosslinkers. Such external crosslinking agents, added to improve hydrophobicity of the coatings stabilized using these hydrophilic additives, can lead to other handling and processing problems, including limited potlife and potential toxicity problems associated with some of the commonly employed crosslinking agents.
U.S. Pat. No. 3,941,733 (Chang) describes dispersions of polyurethane containing pendant water-solubilizing groups and terminated by hydrolyzable or hydrolyzed silyl groups which can form self-supporting films and coatings on webs. Such water-solubilizing groups are introduced to the polyurethane through the reaction of a stoichiometric excess of an isocyanate-terminated prepolymer with a water-solubilizing compound which, in addition to the water-solubilizing group, has two isocyanate-reactive hydrogen atoms. To form higher molecular weight poly(urethane-ureas), Chang internally chain extends with the water to form multiplicities of chain extending urea linkages rather than incorporating externally added chain extenders. Leather coated with one of these compositions has excellent wear-resistance.
The inclusion of water-solubilizing compounds, such as diol acids, in conjunction with salt-forming compounds, such as tertiary amines, in polyurethane compositions has been described by Herman et al., in U.S. Pat. No. 3,640,924. The intermediates are emulsified in the presence of salt-forming compounds and thickeners are added to give curable adhesives.
Polyurethanes have also achieved commercial acceptance in wood finishing systems because of their overall balance of properties, such as abrasion resistance, flexibility, toughness, high gloss, as well as mar and solvent resistance. Early commercial systems were either solvent based reactive high solids prepolymers reacted with a second component, solvent-based moisture curing compositions, or fully reacted urethane lacquers generally dissolved in alcohols and/or aromatic solvents.
In an effort to eliminate solvents and their associated emission and handling problems, waterborne urethane wood coatings were developed. One means established for approaching the performance of solventborne polyurethanes in a waterborne composition has been to add an external crosslinker to the polyurethane dispersions. While these additives do improve the durability of such two-part coatings, crosslinked compositions unfortunately also present problems of limited pot life and potential toxicity due to the chemical nature of many of the standard external crosslinkers (e.g., multifunctional isocyanate and aziridine crosslinking agents).
The primary function of clear wood finishes is to enhance the natural beauty and protect wood surfaces from cumulative weathering effect of sunlight and moisture. Silicone containing finishes are preferred for use in harsh marine applications to protect wood above the water line from exposure to sun, rain, and salt-fog. However, the adhesion of silicones to oil woods, like teak, is poor. For teak, known polyurethane coatings adhere well but experience degradation upon exposure to sunlight. Wood continually undergoes dimensional changes caused by fluctuations in humidity and temperature in the use environment. The rate and magnitude of these changes can be controlled to some degree by the moisture permeability of the coating. Therefore, a wood coating must have sufficient elasticity to expand and contract with the wood, yet have adequate adhesion to resist the interfacial stress generated by the differential movement between the coating and wood surface. Coatings having a low modulus of elasticity will generate less interfacial stress for a given amount of movement than those with a high modulus. Hydrophilic coatings are plasticized by adsorbed water which increases their elasticity and peel tendency.
The weathering of wood proceeds by a series of complex, free-radical chemical reactions. The free radicals are photolytically generated in wood by both ultraviolet (UV) and visible light. Small amounts of moisture (0 to 6%) increase the concentration of free-radicals. These radicals rapidly react with atmospheric oxygen to oxidize (degrade) the wood surface. While UV light stabilized, clear wood finish coatings can slow the ingress of moisture and light and thus retard the degradation process to some degree, eventually the wood at the interface deteriorates causing the coating to flake and chip-off the surface. The rate of these degradation reactions is a function of the finish film composition and moisture adsorption/permeation.